Ed, if you are looking at the various rockers on this site, I would suspect that the concave back rail or comb sections are achieved with a spoke-shave with a slightly curved base. The older classic makers used to offer both variants, flat and curved bases, which may account for your ‘chatter’ if your base is flat, also amplified if the cut angle is straight rather than skewed. The blade would then need to be extended beyond the sole to overcome the curve. The convex backs will work as normal with a flat base.
By ‘heavily set’ I mean to say that the angle of the ‘hook’ on the edge of the scraper blade is well pronounced to give a deeper cut.
When I mentioned the Stanly No: 82, I did not intend to confuse it with the No: 80 pattern which is a different tool entirely. There are plenty of pictures of both on the internet. Sadly, Stanley hasn’t made it since the 1950s, though Veritas brought out something similar….